Distortion correction in wave transmission



Dec. 20, 1938. J. T.'DIXON r AL 5 2,140,900

' DISTOQTION coRREcTIofi IN W-AYE TRANSMISSION IRE." F] L) File d Aug. 10, 1937 .JZ'E.

" g INVENTORS I JTZZWM/QJG. Ewen ATTORN EY phone circuits, in which a plurality of channels the transmission 012 independent message ;,While theamountl-of modulation in-any5=one relarge number, such asi several hundreds-of re modulation which is particularly to be; guarded against is that known as interchannel:modula-' tified as intrachannel modulatiom s minal station the ratio of modulation products to ,signal intensity;

mission system in which a wide frequency'band is divided intoa large number of signal ichan-f' j l nels.- ;In this case the width of any onechannel band is usually small compared to the total sig naling band but repeaters are .used in rcommon by all ora large number "of the channe1s.;; 'I'he non-linearity of ;the .repeaters give riseto sum and diiference frequencies commonly-called modulation frequencies or ;modulation ;products any; one modulation frequency may be obtained in.. numerous way mayta e pn aflar'g'e yariety of values butlaipar -v ticulaijlpalr of 'yalues may begspokengofjas one modulation source for. the frequ ency.ja; and. there is in any specified channel. r V. r

.L In a copending application of Kreer, Serial'No.

60 158,295, filed of even date herewith,'thereis dis cussed the question'of the manner in which modp 55 lc s's'and arepeaterof flat gain, and if there is to such systems as are identified ascarrier tele- -;are ,zusedf.on a singlev transmission medium for;

peater may be small if the system comprises a peaters in.tandem, then the cumulativeeffectf may become substantial. One of the types of tion. Another type is that which may bei der iv V This invention relates to methods and means Y "f'or'maintaining as ,low,.as possible at'the .ter-'- r-As pointedput abovei gtheinventionis espe cially, but not exclusively, applicable tov a vtrans "will manygsuch sources Iin" the broad bandf; giving rise to' a particular modulationfrequency DISTORTION CORRECTION IN WAVE I A TRANSMISSION v John T. DixonrNew York, N. .'Y., and John G. Kreer, Jr., Bloomfield, N. 1., rassignors to Bell Telephonev Laboratories, f x York, N.'. !f., a corporation of New York A Application August-lfl, 1937, No."158,296 I we soiaima; (clima -5'14 This invention-relates to communicationsysj tems and more especially, though not exclusively, j

Incorporated, New

no relative phase shift of the different frequency componentsas one goes through a section of cable,thenit is possible by proper poling of the v repeaters 'to cause the even-power modulation fttermsi to balance out; but: that the odd-power terms -Willaaddup arithmetically,"soithat-if E g is the zmodulation "voltage'generated in each re-- peatermnd'there are n repeaters,then the modu-: lationwoltageat the end cf the linewill'bemE ;for;odd'-:number terms and the modulation power outputflwill' be proportionalzto 21 E, iwhereasithe I zsignalfintensity. will bethe same .;at .the .receiv- V ingisend as rfrat the transmitting t ends-1511b was v ut's inithat application that if a irelative phasershiftiamongthe components of different j frequencyt'is'introducedin each-repeater section; either-i' because of the. phase-frequency characteristic -of the .cable span or because of intentionallyintroduced phase distortion, that the balancing'out' of the evenipower terms will no longer be present -andiallsd that theiodd-power terms nolongeradd up arithmetically. V Also in that application it was pointed out that by a suitable relative phase shift 0 between the modulation product generated-within a repeater section from given fundamental frequencies and the similar modulation product arriving from the r previous section it is possible to causethe modulation gvoltagev generated in therespective repeater' sections to add up 'vectorially. in a" systematic manner and in such a way as to cause .fore.

It was 8151, pointed out that if E is the modulation voltage generated in one repeater from a certain modulation source and if the amount of v voltage modulation :from that source permitted for zthe-lwhole system is E1==at the output, that fj the actualgnodulaticn voltage will not exceed E1 regardlessp of the number of: repeaters, if we fii'intentionally dntroduce phase distortion at each 'repeateriofqsuch nature-as tomakethemodulation phasefincrement at that frequency satisfy therelation' 3 matter of specifying the most useful form of phase frequency distortion in order to attain thedesired results constitutes the chief purpose of this invention, which invention will be better understood by reference to the following specificationl and the accompanying drawing; '1 in Fig. 1 shows a transmission line with a large number of repeaters in tandem, each repeater being provided with a phase equalizing or phase shifting device, the characteristic of which will be described later;

Fig. 2 shows a modification of Fig. 1;

Fig. 3 is a vector diagram to assist in the explanation of our invention;

Figs. 4 and 5 show phase frequency characteristics of the phase equalizers which we find it desirable to give to each repeater section; and.

Fig. 6 represents one form which the phase equalizer may take.

Referring more particularly to Fig. 1, there is shown a transmitting station Ti and a receiving station T2 joined by a transmission line such as, a

coaxial cable. In this line there are numerous repeaters R in tandem. In accordance with well? established practice, the gain of each repeateris... set at such a value as to just compensate for'loss among .the currents of diflerent frequency in-.ac--

cordance'with the plan to be shown herein; In this invention itiis desirable that the sections ineluding the phase equalizer beas nearly identical aspossible. a

The desired condition for the phase. displace-- ment, as it refers to a single modulation frequency arising from a given modulationsource, has been given above. In general, however, as

' already pointed out, there will be a number of sources which will give rise to the same modulation frequency lying in the channel which is under consideration. In that case the total mod ulation product will be given by i I= '2 sin 6r1 J and the condition, to insure that this value shall not, at the end of the line, rise above the permissible modulation voltage is given by the relation l Q r=1n 1 k E 1 v r=1 Sin -Q,

The analysis given above maybe made more clear by the graphical representation of Fig: 3: If the vector AB represents the modulation volt age generated in the first repeater from a particular modulation source as it appears at the" output of the second repeater, then there will be an equal modulation product generated in'the second repeater but because of the phase distortion present between the output of the first and the output of the second repeater this second vector will be out of phase with the first one by the angle 0 and would be represented in Fig. 3 by the vector BC. The resultant of these two vectors obviously is the line joining the points A and C. If additional repeaters are considered, then the vector diagram is obtained by continuing the drawing of the vectors, each with a phase increment 0 and the resultant modulation voltage is given by a vector drawn from the beginning of the first vector to the end of the last vector. It will be seen that the maximum value which the modulation voltage can attain is that given by the diameter of a circumscribing circle, this circle being the one identified as passing through the three points A, B and C. The condition for maximum value of the modulation product in :the situation described analytically heretofore-corresponds to thediameter of the circle and the possibility is immediately envisaged of reducing this actual modulation product to a value substantially below that corresponding to the diameter of the circle. More specifically, it will be seen that in such a system a particular modulation product as a function of the number ofrepeaters alternately reaches maximum and minimum values-with increasing numbers of repeaters, changing in a cyclical manner. It will be noted-:thatnthediameter of the circle is given y sink. H.

This expression relates to any phase frequency characteristic butwe have found that a phase characteristic such as iepresented by curve A of Fig. 4 is a. particularly suitable one for the pur-- pose at hand. In-this Fig. 4 it will be noted that the phase-shift over the transmission band-is related to the frequency by means of a straight line, which extrapolated to zero frequency intersects'at a phase shift different from zero, say we.

Under these conditions any second order modulation product generated by two frequencies within the transmitted band and itself lying within the transmitted band will have a phase increment equal to ilpo. modulation product, except the positive third order differences, will have a phase increment of- :2 m. This can be readily demonstrated'by an illustration. Consider, for example, a third order modulation frequency of 2 1+fz, where f1 and ii are the frequency sources which give rise to this modulation frequency. The equation for the phase shift frequency characteristic of curve A is given by o+kf. On substitution of f=2fi+fa it will be noted that0=2 po. Thus, any third order modulation of the form 213+): from any two sources will have the same relative shift as one movesfrom one repeater to another. So also for modulation products of the form 3f1. Second order modulation frequencies will have a constantphaseshift perrepeater section. of

0= po. l l

In general, the importance of any modulation product decreases as the order of the modulation product increases. Also, so far as the third order modulation products are concerned, all of these call for the same value of 0 with the exception of the positive third order differences. Inasmuch as these latter constitute a relativelysmall portion of the total modulation products they Likewise, any third order so long as it fulfills the condition that a straightline drawn through any three points of the-characteristic' shall not have an intercept numerically smaller'than o."""-- Ordinarilyjt ;-wou l d Qb'e dimcult "to obtain a phase' frequency characteristic in accordance with curve A' of-"ifiig'iiextendingdown to zero some such manner as indicated by the full line of curve A. The frequency F1, below which the this is not a serious limitation for there are other reasons which, make it desirable to.,relegate this lower frequencyband to other uses.' It is of importancefor the best results 1n the tionduring ;operation. Here an equalizer section is takento' mean the line between'two adjacent :phase aequalizerslmlus any. andl al 'Lattachments thereto.

If these become of significance, it is desirable to matic.

duced to such extent as seems desirable in connection with the exercise of this invention. a f

" Having determined the desired form of-phase' frequency characteristic as represented by curve it then becomes feasible to design a network which, taken alone or in combination with the equalizer section, will give the desired phase frequency characteristic. The matter of design of t' such a network does not constitute a part of our invention but the procedure for such design is set forth in the literature in such articles as, for

example, by Zobel in the Bell System Technical Journal, volume 7, page 488, or patent to Zobel 1,603,305 of October 19, 1926. Fig. 6 illustrates one type of network commonly ca.lledan;-fall-.

consist of inductances or capacities or both and h v 146,906 may; fiscally {be t me: If, on the otiir' and? frequency. As one approaches zero frequency,

the characteristic itself would approach zero in for the type of signaling in mind. In practice use of our invention that the successiveequalizer g": .sectionsshall be asv nearly identicaligasipossible" and'that they shall bemaintainedin' this condi -.istic 1 linedrawnthrough any three points on the por-j "'tion -oithe resultant characteristic of line section. plus equalizer within the useful band has a phase interceptat zero frequency at least as great as the phase intercept at zero frequency of a phase In practice, the ideal condition of perfect identity and. constancyfcannot be reached for there will be variations in thesec-f; tions due to numerous causes suchas tempera-" ture or humidity changes or aging of the tubes.--

introduce compensating means, preferably auto Such compensation means have been de-- scribed in the application of Kreer citedabove, 2 and it is contemplated that these would be intro- A of Fig. 4 and represented further by p= po+ f,

pass structure, which has great fiexi'oility so far as phrase frequency characteristic is concerned.' The impedance Z: and Z2 in this network may each impedance may be a simple unit or a com- By proper choice of the capacities and a means hat if the value 'of E'is-the same as'for the second? order iefiects} the -vector "'circle" is smaller inl'diameter; Thus third order-=modulation products are minimized even more effectively than-the second order products. F

fI'hislOpe" of curve 'A of Fig; 4 is given by the Constanta;- Usually' the slope of this curve :will not be 'far -different from that of 7 the curve'B, which is that of a transmission line-section without the equalizer. "2* .5 What is claimed t.

Q 1. In a signal transmission system comprising a transmission line adapted for a wide band of frequencies, a pluralityof-repeaters in tandem in said lin,,means for increasing the was of signal to modulation products for the system comprising phaseequalizing means introduced in the line dividing the line into electrically equal sections,

, the phase frequency characteristic of each equalstraight line characteristic is not maintained, Y, can by suitable design be made quite low and the frequency band from 0 to F1 would not be used .1

izer plus its section being such that a straight line drawn through any three points on the portion of the characteristic lying within the useful band 2. In;a"{signal transmission system comprising Q aktran'smission line adapted for a plurality of channel ;a 'plurality of repeaters in tandemuin said :line o spaced that the repeater sections are electrically equal, means 'IfOi increasing the ratioof. signal to modulation products for the'system comprisingE-a phase equalizer introduced in each epeaterrsection, the phase frequency charactertheiequalizer' being such that a straight 3. Thecombination of claim 1 characterized by the fact that the phase frequency characteristic.

of a section throughout said wide band of frequencies differs from a fixed phase shift at all of said frequencies by an amount proportional to ,the frequency.

- the factfthat the phase frequency characteristic of the-equalizers are substantially straight lines 7 over the eflfective signal band.

6. In asignal transmission system comprising a transmission line adapted .for a plurality of channela ea. plurality of repeaters in tandem in said lineijmeans for increasing the ratio of signal v to modulation products for the system compris- ,ing phase equalizing means introduced in the line at a plurality of intermediate points dividing the *line into'ia plurality of electrically equal sections,

the'phase frequency characteristic of each equalizer plus'its section within the utilized signal frequency band being linear except for departures from the linear represented by sinuosity of such character that a straight line-drawn through any three points of the phase frequency characteristic within the signaling band will have a positive intercept greater than zero.

. 7. In a signal transmission system comprising a transmission line with a plurality of repeaters in tandem; each giving rise to a modulation voltage which from any one modulation source is voltage iorgthe-line at that frequency below a predetermined valueErjn excessof E,- which consists in--intrcducing:in-connection with each repeater section a phase irequencydistortion'such that the phase: frequency characteristic of the section within: the range of utilized frequencies is substantially ofthe. form o+kf-whereo has a value numericaliy equal to or :less thanc and the latter is defined by l a sm -E E 8:11:19. signal; transmission system comprising a:- transmission line adapted ;for a'wide band of gkvgn by. E themefihodaof-keeping the modulation- 

